The grooves of the breech-loader (original) are saw-toothed in shape, vanishing at the shot-chamber for use with a lead-coated projectile. [Illustration: Armstrong Groove for Breech-Loader.] [Illustration: Armstrong Shunt Groove for Muzzle-Loaders.] [Illustration: Armstrong 100-ton Gun (Improved Construction).] The grooves of the muzzle-loader (original) are of the shunt or double type, being cut to the full depth for the entire length and _one half the width_ of the groove. The other half of the width is cut on an incline from the muzzle towards the chamber, this half being the driving side. The loading edge of the groove near the bottom is cut at a sharp angle so as to throw the studs of the projectile against the driving edges of the groove, and give it a close bearing. In coming out the studs ride up the incline of the driving side and are pinched, centring the projectile. This groove is now obsolete. The groove of the improved muzzle and breech-loaders is the plain groove. BREECH-LOADING GUNS (ORIGINAL). _7-inch B. L. R. 82 cwt._ This calibre has been totally withdrawn from service, and has been replaced by the muzzle-loader. The body of the gun consists of an A tube, a breech-piece and B tube, a trunnion-ring and six coils. A vertical slot pierces the gun at the base of the chamber, into which fits a block called the vent-piece. This block is quite light and is easily lifted out or replaced by means of handles. The forward face of the block fits the rear of the chamber, and the escape of gas is prevented by means of a tin-cup gas-check. This vent-piece is held in position when in the gun by a block termed a breech-screw. This block is provided with a full screw-thread, and works longitudinally, the forward end of the block seating against the back of the vent-piece. This breech-screw is provided with a handle for turning, the handle being weighted at the end, and having a lost-motion movement by which the block may be driven close home when screwed up. The vent is bored in the vent-piece, piercing it vertically to the centre, and then turning at right angles. The breech-screw is hollow, the hole being of slightly less diameter than the powder-chamber. When the vent-piece is out, the gun is cleaned and loaded through the breech-screw. [Illustration: Armstrong Breech-Loader.] [Illustration: Vent-Piece.] [Illustration: Breech-Screw.] _40-pdr. B. L. R. 32 and 35 cwt._ The same general type as the larger calibre. In this and smaller calibres there is no separate gas-check, the joint consisting of two bevelled copper facings, one on the vent-piece and the other in the powder-chamber, which form a close contact and seal the joint by expansion. _20-pdr. B. L. R., 15 and 13 cwt.; 12-pdr. B. L. R., 8 cwt.; 9-pdr. B. L. R., 6 cwt.; 6-pdr. B. L. R., 3 cwt._ Same construction as the 40-pdr., except that the coils are fewer. _Modified Breech-Loaders._